1. Field of the Invention
The present invention relates to an image enhancing method and device, and more particularly, to an image enhancing method for enhancing the contrast of an image without an increase in noise and a device for performing the image enhancing method.
2. Description of the Related Arts
A histogram of a gray level provides an overall description of the appearance of an image. Properly adjusted gray levels for a given image can enhance the appearance or contrast of the image.
Among many methods for contrast enhancement, the most widely known is the histogram equalization method, in which the contrast of a given image is enhanced according to the sample distribution thereof. The method is disclosed in documents: [1] J. S. Lim, "Two-dimensional Signal and Image Processing," Prentice Hall, Englewood Cliffs, N.J. 1990, and [2] R. C. Gonzalez and P. Wints, "Digital Image Processing," Addison-Wesley, Reading, Mass., 1977.
Also, the useful applications of the histogram equalization method for medical image processing and radar image processing are disclosed in documents: [3] J. Zimmerman, S. Pizer, E. Staab, E. Perry, W. McCarteney and B. Brenton, "Evaluation of the Effectiveness of Adaptive Histogram Equalization for Contrast Enhancement," IEEE Tr. on Medical Imaging, pp. 304-312, Dec. 1988, and [4] Y. Li, W. Wang and D. Y. Yu, "Application of Adaptive Histogram Equalization to X-ray Chest Image," Proc. of the SPIE, pp. 513-514, vol. 2321, 1994.
In general, since the histogram equalization causes the dynamic range of an image to be stretched, the density distribution of the resultant image is made flat so that the contrast of the image is enhanced as a consequence thereof.
However, the histogram equalization method causes problems in some practical cases. That is, since the contrast is enhanced by stretching the dynamic range, background noise is simultaneously increased by the equalization. Thus, the equalization may cause degradation of image quality in a region having a constant level.